1. Field of the Invention
This invention relates to a device to reduce pulsations on a hydrostatic positive displacement unit such as an axial or radial piston machine which can be used both as a pump and as a motor with a reversible direction of rotation. The displacement unit includes at least one piston mounted so that it can move longitudinally in a cylinder bore which forms a cylinder chamber. The device has a buffer element which can be brought into communication with the cylinder chamber through a connecting channel.
The invention further relates to a device to reduce pulsations on a hydrostatic positive displacement unit which has a pre-compression device which is provided with a valve located between a low-pressure side and a high-pressure side and opens toward the high-pressure side. The pre-compression device creates a connection between the cylinder chamber and the high-pressure side as soon as the pressure in the cylinder chamber exceeds the pressure on the high-pressure side.
2. Background Information
Hydrostatic positive displacement units of this type generally have a plurality of cylinder chambers and deliver a non-constant, pulsating volume flow. One of the causes of these pulsations in the volume flow of the positive displacement unit is the result of the kinematic conditions. On such machines, the hydraulic fluid is transported by several pistons which can move longitudinally in cylinders and work according to the positive displacement principle, from the low pressure inlet side to the high pressure outlet side. As a result of the superimposition of the individual volume flows to form the total volume flow of the positive displacement unit, there is a pulsation in the flow being transported. This type of pulsation is designated a kinematic pulsation.
An additional cause of the pulsations is the kinetic pulsation which originates from the compressibility of the medium being transported, and which occurs primarily when there are large pressure differentials between the inlet side and the outlet side. This type of pulsation is caused by pressure equalization currents which occur during the reversal actions of the cylinder chambers from the inlet side to the outlet side. If, for example, a cylinder chamber of a rotating cylinder drum is moved from the inlet side, which is under low pressure, to the outlet side, which is under high pressure, at the corresponding dead center position of the movement of the piston, the cylinder chamber traverses an area in which the cylinder chamber is briefly not in communication with either the low pressure side or the high pressure side. When communication is subsequently established between the cylinder chamber and the high-pressure side, volume flows occur as a result of the pressure differential between the cylinder chamber and the high-pressure side. As the cylinder chambers move further, the cylinder chamber also traverses an area in which the cylinder chamber is not connected to the high-pressure side or the low-pressure side. Large pressure differentials also occur when it is reversed to the low-pressure side. Consequently, pulsations originate which result in vibrations and noises in the positive displacement unit.
To reduce the pulsation, the prior art teaches measures which provide an adaptation of the pressure in the cylinder chambers to the pressure at an outlet side which is under high pressure by the kinematics of the piston. The adaptation of the pressure can be achieved, for example, by a pre-compression of the cylinder chamber. In this case, a pre-compression device is located between the inlet side and the outlet side, whereby the pressure of the hydraulic fluid contained in the cylinder chambers is increased by the piston stroke, before the cylinder chamber is placed in communication with the outlet side.
A hydraulic axial piston machine of this type is disclosed in DE 33 19 822. The pump has a low-pressure inlet side and a high-pressure outlet side, whereby there is a pre-compression zone between the low-pressure side and the high-pressure side. In this pre-compression zone, there is a connecting channel which creates a connection between the cylinder chambers and the outlet side. Located in this connecting channel is a valve which opens toward the outlet side and prevents the pressure in the cylinder chambers as a result of the pre-compression from increasing to excessive levels which could damage or destroy the machine.
As the cylinder drum moves from the inlet side to the outlet side, the cylinder chamber moves through the pre-compression zone, as a result of which the pressure in the cylinder chamber is increased by the piston stroke. As soon as the cylinder chamber exposes the opening to the connecting channel, fluid flows through the connecting channel and the valve into the outlet side if the pressure in the cylinder chamber exceeds the pressure on the outlet side. The pressure in the cylinders is thus limited to the pressure at the outlet. As the cylinder chamber moves further, it is opened toward the outlet side.
Under these conditions, the valve opens as soon as the pressure resulting from the pre-compression in the cylinder chambers equals the pressure on the outlet side. In positive displacement units with adjustable stroke volumes which work against pressures on different levels on the high-pressure side, the pre-compression zone must be realized so that the pressure can be increased to the maximum operating pressure of the machine. For this purpose, the pre-compression zone between the inlet side and the outlet side must have a corresponding length to make possible a pre-compression to the maximum operating pressure.
If only a low pressure is applied to the outlet side, losses occur on account of the pre-compression of the cylinder chambers, as a result of which the hydrostatic positive displacement unit operates with poor efficiency.
A pressure equalization between the cylinder chambers and the high-pressure side can also be achieved only if the pressure resulting from the pre-compression in the cylinder chambers exceeds the pressure on the high-pressure side and thus opens the valve.
To reduce pulsation, the prior art further discloses the use of a buffer element which effects an equalization between the pressure in the cylinder chambers and the pressure on the high-pressure side.
A hydrostatic machine utilizing the axial piston design with such a buffer reversal is described in DE 42 29 544. On this machine, there is a buffer element in the form of an oil-filled pre-compression volume which is placed in communication with the cylinder chamber after the cylinder chamber has passed the dead center position by means of a connecting channel and an opening in the control plate. Hydraulic fluid is thereby extracted from the pre-compression volume, as a result of which the pressure in the cylinder increases. The pre-compression volume is filled via a line which is in communication with the high-pressure side of the machine.
The pre-compression volume is supplied with fluid via a constant connection between the pre-compression volume and the outlet side of the machine. If a cylinder space moves from the inlet side to the outlet side, and if low pressure is applied to the inlet side and high pressure to the outlet side, hydraulic fluid is extracted from the pre-compression volume as soon as the cylinder chamber has exposed the opening in the control plate. As a result of this measure, the pressure in the cylinder chambers is equalized to the pressure of the outlet side, whereupon lower volume flows are formed to equalize the small remaining pressure difference when the cylinder chambers are connected to the outlet side. With this measure, however, a specially designed cylinder nodule is required to connect the cylinder chambers with the pre-compression volume, to make it possible for the hydraulic fluid to flow rapidly from the pre-compression volume into the cylinder chamber.
The prior art also includes the recharging of the pre-compression volume during the period in which the cylinder space is in communication with the high-pressure side. To fill the pre-compression volume only a temporary communication with the high-pressure side is established. For this purpose, a specially shaped cylinder nodule is required. While the cylinder chamber is in communication with the pre-compression volume, this cylinder nodule first briefly establishes communication between the cylinder chamber and the pre-compression volume. During this period, the pressure in the cylinder is increased. Then the communication is interrupted. In a further phase, an increasingly large cross section is formed which makes it possible to fill the pre-compression volume as soon as the cylinder is in communication with the high-pressure side.
In these measures with a buffer reversal, the cylinder chamber is placed in communication with the pre-compression volume only briefly. Only a short period of time is therefore available for the required pressure equalization. The time during which the cylinder chamber is in communication with the space via the connecting channel is controlled by the geometry of the connecting line and of the cylinder nodule. The optimum opening time must thereby be considered the time during which a pressure equalization can take place between the cylinder chamber and the pre-compression volume. This opening time is a function of the operating parameters, such as the speed of rotation, the operating pressure and the displacement position. The opening time with these measures is defined by the geometry of the components, which means that an effective reduction of pulsations is not achieved under all operating conditions.
A further disadvantage of these measures is that a correspondingly large pre-compression volume must be provided to achieve an effective reduction of pulsations. However, such a large pre-compression volume requires a correspondingly large amount of room in the machine in which it can be located.